Ok, so i know that everyone has their own hc for how old he is, especially considering that we are never told his age, however-
I strongly reject the common notion that legolas is more than a few years younger than elladan and elrohir.
And just to clarify, you can hc what you want, i’m not saying it’s incorrect, this is simply my opinion.
But for me, it makes no sense to send a legit really young elf on the quest, so that already eliminates the idea of Legolas being really young or the youngest elf in middle-earth.
Furthermore, some people might point out that Legolas’s attitude is very carefree and lightharded and seemingly young, to which i point at book!thranduil and say “yeah, so is this one, and he’s probably the same age as elrond”. The wood elves that we have seen in the hobbit act pretty lively, they like to party, and they know how to enjoy themselves even with the looming shadows encroaching on their forests. Is it really that surprising that Legolas has a similar attitude throughout the quest?
If anything, the way we are shown how the wood elves live, how thranduil is, gives Legolas’ own light hearted and joyous attitude a rather solid foundation, and it is a shame that foundation is ripped away by the hobbit movies. (The mis representation of the woodelves is probably my biggest pet peeve of the movies)
Now, you’re probably wondering “ok, well than how old do you think he is?”
To which i answer that he was probably born sometime during the 2nd age, where exactly i don’t know, but here is my reasoning for this time line.
Legolas refers to himself as a silvan in the books, which likely indicates that he was born and grew up in greenwood after Oropher’s people fleed there after the sack of doriath, and that most likely his mother was a silvan.
I’m fairly certain that we can all agree on at least that.
However, the reason i say he was probably born before the 3rd age is largely speculation on my part, and it actually has to do with The Last Alliance.
Now, we know that Oropher and Thranduil and a good chunk of their military rode to join this battle, which eventually killed Oropher and resulted in Thranduil being king.
Someone had to hold down the fort in greenwood, and seeing as we know literally nothing about either of the two’s wives or where they were, i’m inclinded to believe that at least one of them remained. Now, you could argue that it was Oropher’s wife that remained, which leaves me to my next point-
Why would thranduil, the heir of the woodland realm, ride out with his people to his possible death, when Oropher was still alive and kicking?
Legolas was already born, and probably even already an adult by elven standards. That way there would have been an heir kept safe that could lead the people should both Oropher and Thranduil fall.
I don’t think that, with his experiences in doriath and it’s sacking, Oropher would have been ok with his heir riding alongside him to his possible death unless there was someone in place to take over the crown should the worst come to worse, someone who would already be old enough to also take up the duties of the crown and have the experience of leading (albeit only some of) the people that were left behind when the army went to fight.
Now i’m not saying legolas has to be old-old, he could have simply been born only a century or so before the last alliance, but i do think that, due to the reasons i mentioned above, he was born during the second age.
You are, of course, free to think whatever you want, but this is what makes the most sense to me.